1. Field of the Invention
The present invention relates to a skin-covered foamed molded article of propylene resin expanded beads in which substantially the entire surface of the molded article of the expanded beads is covered with the skin. More specifically, the present invention is directed to a skin-covered propylene resin foamed molded article, wherein the molded article of expanded propylene resin beads is placed within the skin which is in the form of a hollow propylene resin molded body obtained by blow molding.
2. Description of the Related Art
A skin-covered foamed molded article is hitherto known, which is obtained by foaming expanded beads, filled in a hollow skin material, for integration with the skin material. One known method for producing such a skin-covered foamed molded article includes the steps of suspending a resin parison between mold halves of a blow mold, closing the mold halves, blow molding the parison into a hollow molded body, filling expanded beads in the hollow molded body, blowing a heating medium such as steam into the hollow molded body to heat and fuse-bond the expanded beads to each other, and cooling the resulting molded assembly to obtain the skin-covered molded article of the expanded beads in which substantially the entire surface of the molded article of the expanded beads is covered with the skin. Such a method is disclosed in, for example, Japanese Patent No. 2,860,007.
Japanese Patent No. 2,860,007 discloses a method for producing a skin-covered foamed molded article by blow molding, in which method expanded beads of a thermoplastic resin are filled in a hollow molded body, produced by blow molding, before the hollow molded body cools and solidifies, and thereafter feeding steam for heating the expanded beads.
In the conventional method for producing a skin-covered molded article of expanded beads having a skin formed by blow molding, it is necessary that the method can produce a skin-covered molded article of expanded beads having excellent bonding strength between the skin in the form of a hollow molded body and the molded article of expanded beads and between the adjacent expanded beads in order for the skin-covered molded article of the expanded beads to exhibit excellent mechanical properties such as bending strength. In order to produce such a skin-covered molded article of expanded beads, it has been considered to be necessary to fill the expanded beads of a thermoplastic resin within the hollow molded body before the hollow molded body cools and solidifies, as in the method described in Japanese Patent No. 2,860,007, and to control the thickness of the skin to more than 3.5 mm so as to prolong the time required for the skin to cool and solidify.
Whilst a skin-covered molded article of expanded beads having high bonding strength between the skin and the molded article and, therefore, excellent mechanical properties can be obtained when the skin has a large thickness, the weight of such a skin-covered molded article of expanded beads becomes so large that there is caused a problem that lightness in weight, which is one of the important features of foamed molded articles, is adversely affected. There also arises a problem that the molding cycle time becomes long due to the large thickness of the skin. In Japanese Patent No. 2,860,007, the thermoplastic resin expanded beads must be filled in the hollow molded body before the hollow molded body cools and solidifies in order to bond the expanded beads to their adjacent skin surface. Namely, after the hollow molded body has cooled and solidified, it is difficult to bond the skin to the expanded beads during the succeeding step of fuse-bonding the expanded beads to each other by feeding steam for heating.
It is, therefore, an object of the present invention to solve problems in conventional skin-covered molded articles of expanded propylene resin beads with respect to their lightness in weight and molding cycle time which have hitherto remained unsolved in a fully satisfactory manner, and to provide a skin-covered molded article of expanded propylene resin beads having excellent fusion bonding both between the skin and the molded article of expanded beads and between the expanded beads.
With a view toward solving the above problems, the present inventors have conducted a versatile study on base resins for forming skins and on expanded beads from various points of view. As a result it has been found that the desired object can be fulfilled by using a skin formed with a base resin having specific thermal properties. The present invention has been made based on this finding.